Method of manufacturing rotation sensor

Abstract: Provided is a method of manufacturing a rotation sensor which can effectively prevent a sensor element portion from movement during resin molding while forming a resin portion covering the sensor element part with excellent waterproofness and airtightness, in formation of the resin portion by resin molding. Resin is injected through a first injection step in such a state that an end of a bobbin (5) is fixed by a slide core (52), and thereafter the slide core (52) is extracted so that resin is again injected in a vacated portion, which is defined by extraction of the slide core, through a second injection step.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a rotation sensor, and more particularly, it relates to a method of manufacturing a rotation sensor which is applied to a wheel speed sensor for an automobile or the like. Description of the Background Art In general, a wheel speed sensor is known as a sensor for detecting the wheel speed of an automobile or the like. High airtightness is required for such a wheel speed sensor, in consideration of the environment of its application. In general, therefore, an internal detecting portion (sensor element portion) of the wheel speed sensor is inserted in a case of stainless steel or the like, and an opening of this case is sealed with resin. This structure is disclosed in Japanese Patent Laying-Open No. 63-235811 (1988), for example. Fig. 10 is a sectional structural diagram showing the conventional wheel speed sensor which is disclosed in this literature. Referring to Fig. 10, a sensor element portion which is formed by a magnet 102, a coil 103, a magnetic pole piece 104, a bobbin 105 and the like is stored in a case 108 of stainless steel or the like in this conventional wheel speed sensor (rotation sensor). The interior of the case 108 is molded with resin by a first resin portion 109a to a portion which is close to an opening 112 of the case 108. Both ends of a thin wire which is wound on the coil 103 are soldered to lower portions 106a of terminals 106, while these lower portions 106a are covered with the first resin portion 109a. A bracket 111 for fixing the rotation sensor is bonded to an outer side of the case 108 by brazing. Forward end portions 113 of an output wire 107 are joined by caulking to upper portions 106b of the terminals 106. A pipe-shaped output wire protecting member 110 is provided around a portion of the output wire 107 which is close to the forward end portions 113. The opening 112 of the case 108 and the upper portions 106b of the terminals 106 are covered with a second resin portion 109b. The second resin portion 109b is molded under a high temperature and a high pressure, so that the opening 112, the output wire protecting member 110 and a surface of the bracket 111 which is close to the output wire 107 are in close contact with the second resin portion 109b respectively. In the conventional rotation sensor shown in Fig. 10, however, it is difficult to reduce the product cost since the case 108 is made of a high-priced metal such as stainless steel. In general, therefore, there has been proposed a system of covering the overall sensor element portion with resin without employing the case 108 of stainless steel or the like. In such a system of covering the overall sensor element portion with resin, however, the sensor element portion disadvantageously moves by a pressure which is applied for molding the exterior of the sensor element portion with resin. In order to prevent such inconvenience, there has generally been proposed a method of exposing a part of a bobbin, which is one of components forming a sensor element portion, to the exterior of a resin portion and fixing the sensor element portion through the exposed part of the bobbin. This method is disclosed in Japanese Utility Model Laying-Open No. 61-181321 (1986), for example. Fig. 11 is a sectional structural diagram showing a rotation sensor which is disclosed in this literature. Referring to Fig. 11, the conventional rotation sensor is so structured that an end 214a of a bobbin 214 is exposed from a resin portion 220 entirely covering a sensor element portion. Due to this structure, it is possible to fix the sensor element portion through the end 214a of the bobbin 214 in molding of the resin portion 220. In the structure shown in Fig. 11, the sensor element portion is formed by a ferrite magnet 212, a spacer 213, a pole piece 211, the bobbin 214, and a coil 215. In the conventional structure shown in Fig. 11, however, water permeates through a clearance between the end 214a of the bobbin 214 and the resin portion 220 in actual employment, although the sensor element portion can be prevented from movement in molding of the resin portion 220. As the result, insulation resistance in the interior is deteriorated to reduce performance. As to permeation of water through a clearance between the pole piece 211 and the resin portion 220, on the other hand, it is possible to readily improve waterproofness by inserting an O-ring around the pole piece 211 or the resin portion 220. Further, a method of fixing a sensor element portion by small pins in resin molding is also known in general, as another method of preventing the sensor element portion from movement in resin molding. In this method, however, holes are defined in parts of the resin portion receiving the pins which are employed in resin molding. Although the holes are filled up by resin potting or the like in a later step, water still permeates through the filled-up portions of the holes, to cause a problem which is similar to that in the structure shown in Fig. 11. Namely, insulation resistance in the interior is deteriorated to reduce performance by permeation of water in the interior of the resin portion (sensor element portion). Further, the operation for filling up the holes disadvantageously increase the number of steps. FR-A-2 576 245 discloses a method for encapsulating a rotation sensor including a coil. The assembled sensor is put into a mold and centered therein by a bush. The bush is removed during injection of the resin into the mold. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing a rotation sensor, which can effectively prevent a sensor element portion from permeation of water in formation of a resin portion for enclosing the periphery of the sensor element portion. According to the present invention, a method of manufacturing a rotation sensor, which is adapted to manufacture a rotation sensor including a sensor element portion having a magnet, a magnetic pole piece, a bobbin and a coil, and a resin portion with a portion of small thickness and a portion of large thickness enclosing the periphery of the sensor element portion, comprises a step of forming the sensor element portion, a step of inserting the sensor element portion in a resin mold while bringing a bush belonging to the resin mold into contact with the bobbin at a position where the resin portion is of large thickness, a first injection step of injecting resin into the resin mold through its injection port, a step of extracting the bush after the first injection step, and a second injection step of injecting resin through the injection port of the resin mold again thereby filling up a vacated portion, which is defined by extraction of the bush, with resin. The injection port of the mold may be provided around the bush. Further, the injection port of the mold may be so provided that the bush is positioned on its extension. Further, the bush may be extracted simultaneously with completion of the first injection step. Alternatively, the bush may be extracted after a lapse of a prescribed time from completion of the first injection step. In this method of manufacturing a rotation sensor, the bush belonging to the resin mold is brought into contact with the bobbin forming the sensor element portion so that resin is injected into the resin mold in this state through the first injection step, and thereafter the bush is extracted so that resin is again injected into the vacated portion, which is defined by extraction of the bush, through the second injection step. Namely, the bush is extracted after the resin which is injected through the first injection step starts to solidify so that resin is further injected into the vacated portion, which is defined by extraction of the bush, through the second injection step, whereby the sensor element portion will not move during resin molding while the resin molding is readily carried out to seal the sensor element portion. Consequently, the sensor element portion is effectively prevented from permeation of water from the exterior of the resin portion covering the sensor element portion in actual employment. When the injection port of the mold is provided around the bush, further, solidification of the resin is hard to progress around the bush after the first injection step and the bush can be extracted after the remaining portion of the resin starts to sufficiently solidify, whereby the sensor element portion is further effectively prevented from movement after extraction of the bush. When the injection port of the mold is so formed that the bush is positioned on its extension, further, resin is so injected that the same strikes the bush in a high temperature state immediately after injection in the first injection step. Thus, the temperature of the bush is so increased that solidification of the resin is further suppressed around the bush. A structure of a rotation sensor manufactured by the method according to the invention may comprise a sensor element portion including a magnet, a magnetic pole piece, a bobbin which is so formed as to store a part of the magnetic pole piece and the magnet therein, and a coil which is wound on an outer peripheral portion of the bobbin, and a resin portion which is formed to enclose the periphery of the sensor element portion excluding a forward end of the magnetic pole piece for sealing the same. In this structure of a rotation sensor, the resin portion which is formed to seal the sensor element portion is formed by a first resin portion, having a first thickness, covering a surface of the coil, and a second resin portion, having a second thickness of at least about four times the first thickness, covering an upper portion of the bobbin, whereby cooling/solidification by a mold is reduced in the second resin portion having a larger thickness. Thus, solidification of resin is retarded around a bush which is positioned in the second resin portion. A metal support portion for supporting/fixing the sensor element portion and the resin portion is provided to be embedded in the resin portion at a prescribed space from the sensor element portion while resin forming the resin portion is filled up in the space between the metal support portion and the sensor element portion, whereby the sensor element portion is effectively prevented from permeation of water from the exterior through the metal support portion in actual employment, and high waterproofness as well as airtightness can be attained. The bobbin is provided with a cavity for serving as a positioning member in formation of the resin portion. The cavity may be formed on an upper surface of the bobbin. Further, the cavity may be formed on a side surface part of the bobbin which is upward beyond the position receiving the coil. According to this structure of a rotation sensor, the bobbin forming the sensor element portion is provided with the cavity for serving as a positioning member in formation of the resin portion, whereby it is possible to position the bobbin in a self-adjusting manner by inserting a bush into the cavity of the bobbin in resin molding. Thus, positioning accuracy for the sensor element portion is further improved. When the cavity is formed on the upper surface of the bobbin, a slide core may be provided only in a single portion and hence the structure of a mold is simplified. When the cavity is formed on a side surface part which is upward beyond the position of the bobbin receiving the coil, further, it is possible to readily approach the slide core and a resin injection port to each other due to the structure of the mold. Thus, the resin is injected around the bush in a high temperature state immediately after injection when the resin is injected through the injection port, whereby the temperature of the bush is increased to suppress solidification of the resin around the same. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing a wheel speed sensor for illustrating a first embodiment of a method of manufacturing a wheel speed sensor according to the present invention; Fig. 2 is a sectional view showing a resin mold employed for manufacturing the wheel speed sensor shown in Fig. 1; Fig. 3 is a sectional view showing a wheel speed sensor for illustrating a second embodiment of the method of manufacturing a wheel speed sensor according to the present invention; Fig. 4 is a sectional view showing a sensor element portion for illustrating a structure of a wheel speed sensor manufactured according to the invention; Fig. 5 is a sectional view showing a wheel speed sensor including the sensor element portion shown in Fig. 4; Fig. 6 is a sectional view showing a wheel speed sensor for illustrating a structure of a wheel speed sensor manufactured according to the invention; Fig. 7 is a cross-sectional view showing a mold and a molding employed for manufacturing the wheel speed sensor shown in Fig. 6; Fig. 8 is a longitudinal sectional view of the mold and the molding appearing in Fig. 7, taken along the line 300 - 300; Fig. 9 is a partial sectional view illustrating a bobbin 85 shown in Figs. 7 and 8; Fig. 10 is a sectional view showing a conventional wheel speed sensor; and Fig. 11 is a sectional view showing a conventional improved wheel speed sensor. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are now described on the basis of the drawings. With reference to Figs. 1 and 2, a first embodiment of a method of manufacturing a wheel speed sensor is described. First, a bobbin 5 storing a magnet 2, a yoke 6 and a magnetic pole piece 4 therein and having terminals (not shown) embedded in its prescribed portions is formed by resin molding. This bobbin 5 is prepared from PPS resin, for example. Then, a coil 3 is wound on the bobbin 5. Both end portions (not shown) of the coil 3 are connected to the terminals (not shown). An upper end (not shown) of a cable 7 which is inserted in a cable protecting member 10 is connected to the aforementioned terminals. Then, the bobbin (sensor element portion) 5, storing the magnet 2, the yoke 6 and the magnetic pole piece 4 therein and having the coil 3 wound on a part of its outer periphery, and the cable 7 are arranged in a lower mold section 53 and an upper mold section 54. At this time, a forward end portion 4a of the magnetic pole piece 4 is inserted in a cavity 53a of the lower mold section 53, thereby fixing the forward end side of the sensor element portion. Then, a slide core (bush) 52 belonging to the upper mold section 54 is inserted in the mold so that its forward end portion is in contact with a rear end surface of the bobbin 5, thereby fixing the rear end of the bobbin 5. In this state, resin such as 66 nylon, for example, is injected through a gate (injection port) 55. The injected resin starts to solidify from a portion having a small thickness such as that around the outer periphery of the coil 3. Therefore, progress of solidification is retarded in the vicinity of the rear end of the bobbin 5 (around the slide core 52) having a large thickness, which is close to the gate 55 as compared with other portions. When solidification of the resin progresses to such a degree that the bobbin 5 is not displaced even if the same is not pressed by the slide core 52 while the resin is still in a molten state in the vicinity of the rear end of the bobbin 5, the slide core 52 is rearwardly extracted. Due to such extraction of the slide core 52, resin newly flows into a vacated portion 8a, which is defined by the extraction of the slide core 52, through the gate 55. Thus, the vacated portion 8a is filled up with the resin. Thus completed is a resin injection step. After the resin entirely solidifies, wheel speed sensor 1 as resin-molded is taken out from the mold sections 53 and 54. In the method of manufacturing a wheel speed sensor according to this embodiment, the resin injection step is divided into two stages as hereinabove described, whereby it is possible to readily mold a resin portion 8, having high waterproofness, covering the sensor element portion while effectively preventing the sensor element portion from movement in resin molding. Namely, resin is injected into the mold while the bobbin 5 is fixed by the slide core 52 in the first injection step. Then, the slide core 52 is extracted in such a state that the resin as injected solidifies to prevent the bobbin 5 from movement while the same is still in a molten state in the vicinity of the slide core 52. Then, molten resin is introduced into the vacated portion 8a which is defined by extraction of the slide core 52 in a second injection step. The resin introduced into the vacated portion 8a which is defined by extraction of the slide core 52 is integrated with that remaining around this portion still in a molten state, followed by gradual progress of solidification. Thus, it is possible to ensure airtightness after solidification of the resin with no boundary defined between the resin injected in the second injection step and that injected in the first injection step. Consequently, it is possible to form the resin portion 8 with high waterproofness. Further, the bobbin 5 is fixed by the slide core 52 or the resin injected in the first injection step, and hence the same will not move during resin molding dissimilarly to the prior art. The wheel speed sensor 1 shown in Fig. 1 is completed by such a manufacturing method. While water permeating through a clearance between the magnetic pole piece 4 and the resin portion 8 is inhibited from further permeation at a portion where a flange 5a provided on the forward end of the bobbin 5 engages with the resin portion 8, it is further effective to previously coat the flange 5a with a sealing agent of polyamide or the like so that the resin portion 8 is thereafter molded, in order to further improve the waterproofness. The function of the wheel speed sensor 1 is now described with reference to Fig. 1. In actual employment, the wheel speed sensor 1 is mounted in the vicinity of a sensor rotor 80 which is rotated with a tire of an automobile. Magnetic flux which is generated from the magnet 2 provided in the wheel speed sensor 1 passes through the magnetic pole piece 4, to reach the sensor rotor 80. When the sensor rotor 80 is rotated following rotation of the tire, the magnetic flux passing through the magnetic pole piece 4 is modulated in response to the irregular shape of the sensor rotor 80. An output voltage is generated in the coil 3 in proportion to time change of such modulation. This output voltage is taken out to the exterior through the cable 7. Then, an external arithmetic unit (not shown) which is provided in the exterior calculates the rotation speed from the frequency of the output voltage. Referring to Fig. 3, a wheel speed sensor 11 according to a second embodiment of the inventive method has an appearance which is similar to that of the wheel speed sensor 1 according to the first embodiment shown in Fig. 1. For convenience of illustration, Fig. 3 shows a structure taken along a section which is at 90° with respect to that corresponding to the section shown in Fig. 1. In the method according to the second embodiment, a bobbin 5 is transversely fixed by slide cores 62a and 62b in resin molding, dissimilarly to the first embodiment. The slide cores 62a and 62b are extracted after a first injection step, and resin is again injected into vacated portions 18a and 18b which are defined by extraction of the slide cores 62a and 62b. Thus, it is possible to form a resin portion 18 having high waterproofness while effectively preventing the bobbin 5 from movement during resin molding, similarly to the first embodiment. Terminals 9a and 9b provided on an upper portion of the bobbin 5 and end portions 7a and 7b of a cable 7 appear on the section shown in Fig. 3, dissimilarly to the section shown in Fig. 1. Referring to Figs. 4 and 5, a truncated-conical cavity 25a is formed in a rear end surface of a bobbin 25 forming a sensor element portion in a wheel speed sensor 21 of a wheel speed sensor manufactured according to the invention. Thus, it is possible to readily position the bobbin 25 in formation of a resin portion 28 covering the bobbin 25. In the structure, the truncated-conical cavity 25a is formed in the bobbin 25, while a forward end 72a of a slide core 72 is formed to have the same inclination as the cavity 25a. When the slide core 72 is inserted in a mold to position the bobbin 25, therefore, it is possible to automatically carry out positioning adjustment even if the slide core 72 is slightly misaligned with the bobbin 25. Consequently, it is possible to further correctly position the bobbin 25 in formation of the resin portion 28 covering the bobbin 25. Further, the mold is simplified in structure since the slide core 72 may be provided on a single portion. In a vacated portion 28a which is defined by extraction of the slide core 72 after a first injection step, resin is injected through a second injection step. Thus, it is possible to readily form the resin portion 28, which is excellent in airtightness and waterproofness, covering the bobbin 25 having the cavity 25a. Referring to Fig. 6, a wheel speed sensor 31 manufactured according to the invention of a wheel speed sensor has a sensor mounting bracket 40 of a metal such as stainless steel, for strongly fixing the wheel speed sensor 31 in the vicinity of an object of detection. The sensor mounting bracket 40 is integrated into a resin portion 38 by insertion molding. The wheel speed sensor 31 according to the second embodiment is so structured that a clearance is defined between the sensor mounting bracket 40 and a bobbin 85 when the metal sensor mounting bracket 40 is integrated into the resin portion 38. This clearance is filled up with resin 38c along the overall periphery. Due to such a structure, it is possible to effectively prevent water permeating through the sensor mounting bracket 40 from reaching the bobbin 85. Consequently, it is possible to ensure airtightness of a sensor element portion (internal structure part). With reference to Figs. 7 and 8, a method of manufacturing the wheel speed sensor 31 shown in Fig. 6 is now described. Fig. 8 shows a section of the wheel speed sensor 31 appearing in Fig. 6, which is rotated about a magnetic pole piece 4 by 90°. First, a mold 81 employed in this method is so structured that slide cores 82a and 82b are horizontally movable along this mold 81. Gates 83a and 83b are provided to be flush with the slide cores 82a and 82b. Namely, mouths 83c and 83d of the gates 83a and 83b are directed to the slide cores 82a and 82b respectively. The wheel speed sensor 31 shown in Fig. 6 is manufactured through the mold 81 having such a structure. In more concrete terms, the bobbin 85 storing a magnet 2, a yoke 6 and a magnetic pole piece 4 therein and having terminals 9a and 9b embedded in its prescribed portions is formed by resin molding. This bobbin 85 is prepared from PPS resin, for example. Cavities 85a and 85b are provided in both upper side surfaces of the bobbin 85 for serving as positioning members when the bobbin 85 is supported by the slide cores 82a and 82b. Fig. 9 is a sectional view showing the bobbin 85 which is formed by such resin molding. A coil 3 is wound on the bobbin 85, and both ends of the coil 3 are connected to the terminals 9a and 9b. Thereafter ends of a cable which is inserted in a cable protecting member (not shown) is connected to the terminals 9a and 9b. Then, the bobbin (sensor element portion) 85 which is formed in the aforementioned manner and the cable (not shown) are arranged in the mold 81 shown in Figs. 7 and 8. Thereafter the slide cores (bushes) 82a and 82b belonging to the mold 81 are inserted in the mold 81, so that forward ends of the slide cores 82a and 82b are brought into contact with the cavities 85a and 85b of the bobbin 85. Thus, the bobbin 85 is fixed. In this state, resin 88 such as 66 nylon, for example, is injected into the mold 81 through the gates (injection ports) 83a and 83b. The resin 88 as injected first strikes the slide cores 82a and 82b, to be thereafter diffused around the same. Thus, the resin 88 strikes the slide cores 82a and 82b in a high temperature state immediately after injection, thereby increasing the temperatures of the slide cores 82a and 82b. Consequently, cooling/hardening of the resin 88 is suppressed around the slide cores 82a and 82b. The resin 88 flows at the maximum around the gates 83a and 83b, whereby the resin 88 is suppressed from hardening around the slide cores 82a and 82b also by this. Further, parts of the resin 88 around the positions of the slide cores 82a and 82b are larger in thickness by about five times than a part covering the coil 3, whereby thermal capacity is increased in the positions of the slide cores 82a and 82b. Consequently, hardening of the resin 88 is retarded also in these parts. Namely, the resin 88 starts to harden from parts having small thicknesses such as the part covering the coil 3 and that between the bobbin 85 and the metal bracket 40 in this embodiment. In more concrete terms, the resin 88 is completely filled up after a lapse of three seconds from starting of injection, and starts to harden from the parts having small thicknesses after a lapse of six seconds. After the parts of the resin 88 having small thicknesses start to harden, the slide cores 82a and 82b are extracted while the resin 88 flowing around the same is still in an unhardened state. In more concrete terms, the slide cores 82a and 82b are extracted after a lapse of three seconds from completion of resin injection. Thereafter the resin 88 is continuously injected into vacated portions which are defined by extraction of the slide cores 82a and 82b. Thus, it is possible to form a protective resin portion which is integrally joined with the resin injected in the initial stage. While the slide cores 82a and 82b are extracted after a lapse of three seconds from filling of the resin in this embodiment, the present invention is not restricted to this but resin may already start to harden in a stage of completion of resin injection depending on the shape of the wheel speed sensor. In this case, the slide cores 82a and 82b may be extracted simultaneously with completion of resin injection with no problem. While the time for extracting the slide cores 82a and 82b is decided on the basis of starting of resin injection in this embodiment, the present invention is not restricted to this but the time for extracting the slide cores 82a and 82b can alternatively be decided on the basis of a stage when movement of a cylinder for injecting the resin is stopped or a stage when a pressure, such as a pneumatic pressure, for example, employed for injecting the resin is increased in excess of a constant level. According to the method of manufacturing a rotation sensor in present invention, the resin is injected into the resin mold through its injection port in the first injection step while the bush belonging to the resin mold is brought into contact with the bobbin and thereafter the bush is extracted so that resin is again injected into the vacated portion, which is defined by extraction of the bush, in the second injection step, whereby it is possible to readily manufacture a rotation sensor which is excellent in airtightness and waterproofness while effectively preventing the sensor element portion from movement during resin molding. Further, solidification of the resin injected in the first injection step is retarded in the vicinity of the bush as compared with other parts when the injection port of the resin mold is provided in the vicinity of the bush, whereby it is possible to readily extract the bush while the sensor element portion is sufficiently fixed by the precedingly solidifying parts of the resin. Consequently, it is possible to effectively prevent the sensor element portion from movement during resin molding dissimilarly to the prior art. When the injection port of the mold is so formed that the bush is positioned on its extension, the resin strikes the bush in a state of a high temperature immediately after injection, to increase the temperature of the bush. Thus, hardening of the injected resin is further retarded in the vicinity of the bush as compared with other parts, whereby the bush can be readily extracted after hardening of the other parts of the resin. The metal support portion for supporting/fixing the sensor element portion and the resin portion is embedded in the resin portion at a prescribed space from the sensor element portion while the clearance between the metal support portion and the sensor element portion is filled up with the resin forming the resin portion to ensure waterproofness of the sensor element portion, whereby water permeating through the metal support portion will not reach the sensor element portion. The cavity for serving as a positioning member in formation of the resin portion for the bobbin forming the sensor element portion is so provided that it is possible to automatically adjust the sensor element portion at a correct position by the bush even if the sensor element portion is slightly displaced. Consequently, it is possible to further correctly position the sensor element portion in formation of the resin portion covering the sensor element portion. When the cavity is formed in the upper surface of the bobbin, the slide core can be provided only in one portion, whereby the mold structure can be simplified. When the cavity is formed on the upper side surface of the bobbin, further, it is possible to approach the slide core to the resin injection port on the structure of the mold. Thus, it is possible to further suppress hardening of the resin around the slide core. The resin portion is formed by the first resin portion, having the first thickness, covering at least the surface of the coil and the second resin portion, having the second thickness of at least about four times the first thickness, covering the upper portion of the bobbin, whereby thermal capacity is increased in the second resin portion having a larger thickness as compared with the first resin portion having a smaller thickness. Thus, hardening of the resin is retarded in the second resin portion as compared with the first resin portion. Consequently, resin hardening is retarded in the vicinity of the bush which is positioned in the second resin portion, whereby the bush can be readily extracted after preceding hardening of other resin parts. Thus, it is possible to effectively prevent the sensor element portion from movement during resin molding, dissimilarly to the prior art. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A method of manufacturing a rotation sensor (1) including a sensor element portion having a magnet (2), a magnetic pole piece (4), a bobbin (5) and a coil (3), and a resin portion with a portion of small thickness and a portion of large thickness enclosing the periphery of said sensor element portion, said method comprising: a step of forming said sensor element portion; a step of inserting said sensor element portion into a resin mold (53, 54) while bringing a bush (52) belonging to said resin mold (54) into contact with said bobbin (5) at a position of the rear end of said bobbin (5) where the resin portion is of large thickness; a first injection step of injecting resin into said resin mold (53, 54) through its injection port (55); a step of extracting said bush (52) after said first injection step and when the solidification of the resin portion of small thickness has progressed to such a degree that the bobbin (5) is not displaced in the following step; and a second injection step of injecting resin through said injection port (55) of said resin mold (53, 54) again, while said resin injected in the first injection step is still in a molten state in the vicinity of the rear end of said bobbin (5), for filling up a vacated portion, being defined by extraction of said bush (52), with said resin. A method of manufacturing a rotation sensor in accordance with claim 1, characterized in that said injection port (55) of said mold (53,54) is provided around said bush (52). A method of manufacturing a rotation sensor in accordance with claim 2, characterized in that said injection port (55) of said mold (53,54) is so provided that said bush (52) is positioned on its extension. A method of manufacturing a rotation sensor in accordance with claim 1, characterized in that said bush (52) is extracted simultaneously with completion of said first injection step. A method of manufacturing a rotation sensor in accordance with claim 1, characterized in that said bush (52) is extracted after a lapse of a prescribed time from completion of said first injection step.

Inventors: KUME MASAHIRO, SAITO HIDETOSHI, KUME, MASAHIRO, SAITO, HIDETOSHI, KUME, MASAHIRO, C/O ITAMI WORKS OF SUMITOMO, SAITO, HIDETOSHI, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1994
Main classification: G01P1
IPC codes: B29C45, G01P1